Alumina has hitherto been used as an IC packaging material or substrate material. However, with the increasing integration, speed and output power of, e.g., LSI, it has become necessary to efficiently dissipate heat generated in a chip out of the system, and materials which have higher thermal conductivity and more excellent heat dissipation than alumina are being demanded.
Since aluminum nitride not only has high thermal conductivity but is excellent in electrical characteristics such as insulation resistance, dielectric strength and dielectric constant and mechanical characteristics such as strength, it is now attracting special attention as a packaging material or substrate material best adapted for heat dissipation.
As the process for preparation of an aluminum nitride powder, there have hitherto been known the following two nitriding methods: (1) a direct nitriding method for nitriding a metallic aluminum powder upon heating in a nitrogen-containing atmosphere, and (2) a reductive nitriding method for heating a mixture of alumina or alumina hydrate and carbon in a nitrogen-containing atmosphere. Though the aluminum nitride powder obtained in the former method is a readily moldable powder having a high tapped density, it contains relatively high amounts of cation impurities such as iron and, hence, it is not always satisfactory in order to obtain a sintered body having high thermal conductivity. Accordingly, the aluminum nitride powder synthesized by the latter reductive nitriding method is most likely to be a powder for obtaining high thermal conductivity substrates. However, though the aluminum nitride powder obtained by the reductive nitriding method has such an advantage that the amounts of oxygen and cation impurities such as iron are low, it has a low tapped density so that it is difficult to mold.
In addition, the aluminum nitride powder is readily hydrolyzable and involves a problem that a special attention should be paid for storage or handling. Moreover, since the surface of the aluminum nitride powder is usually oxidized or hydrolyzed whereby it is covered by a very thin oxide film layer, even when the aluminum nitride powder is sintered under atmospheric pressure as it is, a high thermal conductivity sintered body cannot be obtained. Therefore, the aluminum nitride powder is usually sintered upon addition of a compound such as calcium carbonate, strontium carbonate, or yttrium oxide, whereby a high thermal conductivity sintered material is obtained. However, this method involves a problem that it is not easy to uniformly mix and disperse such an added compound because the surface characteristics of the aluminum nitride powder are not always identical to those of the compound added.
Furthermore, the aluminum nitride powder is so bulky that if the tapped density is low, it is hard to not only disperse it in a solvent but also obtain a highly dense molded article therefrom. Thus, the degree of shrinkage is high during sintering, and a sintered material with good dimensional precision can hardly be obtained. Still further, if the mixing and dispersion of the aluminum nitride powder and additive powder are insufficient and the homogeniety is poor, the sintering is unlikely to proceed uniformly so that the shrinkage during the sintering is not uniform, resulting in easy formation of warpage of a sintered body or anisotropy of the shrinkage.